Solvent recovery process



y 19, 1966 J. D. BUSHNELL ETAL 3,261,778

SOLVENT RECOVERY PROCESS Filed Sept. 27, 1962 SOLVENT STORAGE l3 ,WTREATING TOWER SOLVENT SOLVENT 25w RECOVERY RECOVERY 5 FURNACE 49 7 Iii l8 20 2 '1 42 FURNACE PRODUCT 22 -45 STORAGE 2 so 4a A 4? 1 ss M 32 2? EXTRACTION STORAGE FURNACE ClRCULATING on. 4 1* STORAGE TANK mumm- FEED STORAGE f L'L'VIrtl'b 39 3e 35 PATENT ATTORNEY United States Patent 3,261,778 SOLVENT RECOVERY PROCESS James D. Bushnell, Berkeley Heights, John L. Ehrler, Landing, and Harold N. Weinberg, East Brunswick, N.J., assignors to Esso Research and Engineering Company, a corporation of Delaware Filed Sept. 27, 1962, Ser. No. 227,124 4 Claims. (Cl. 208-321) The present invention relates to an improved method for preventing the escape of solvent vapors, and providing for their recovery. The invention is concerned with an improved process for solvent treating hydrocarbon oil fractions and particularly relates to the recovery of volatile solvents. Specifically, the invention relates to an improved method of preventing air and water pollution by condensing hot solvent vapors through direct contact of the vapors with a liquid hydrocarbon oil.

In refining hydrocarbon oil fractions, frequently a high temperature, high pressure treating step is required wherein apparatus is protected from excessive pressures by suitable pressure release safety valves. In the event of an upset, vaporsare released through the safety valve to some suitable disposition. Release of these vapors to the atmosphere or to water streams can cause serious pollution problems and loss of expensive treating fluids.

A particular system in which a loss of vaporizable sol vent is a major problem is in the selective extraction of aromatic components from lubricating oil fractions. This process uses a solvent which selectively dissolves the aromatic components in preference to paraffinic components. An example of a suitable solvent is phenol. The lubricating oil fraction is contacted with liquid phenol in an extraction tower. Extract and rafiinate phases from the treated tower are separated and each is heated and the solvent is vaporized and removed in solvent recovery towers. The solvent vapors are then condensed and stored for reuse. The solvent recovery equipment is protected from excessive pressures by safety valves. If a safety valve were allowed to discharge vapor to the atmosphere, a serious air pollution problem would result, in addition to losing the valuable phenol. The conventional way of preventing escape of phenol vapors discharging from a safety valve is to discharge these vapors through a condensable blowdown system. In this system, circulating cooling Water condenses vapors by direct contact. The resulting phenolic water, however, can constitute a disposal problem because many governments forbid discharging of any stream containing phenol into local rivers, streams, etc.

In the present invention, a novel system has been devised to handle hot phenol vapors discharged from a safety valve. The invention may be best understood by discussing it in relation with the phenol treating step in refining a lubricating oil fraction.

A suitable hydrocarbon feed is countercurrently contacted with phenol solvent which preferentially dissolves aromatic components. The hydrocarbon feed separates into an extract and a rafiinate phase. The extract phase contains the phenol and most of the aromatic components and the raflinate phase contains most of the paraffinic components. The majority of the phenol leaves the treating tower with the extract phase. The phenol treating tower is operated at a pressure of about 150200 p.s.i.g. and a temperature of about l50250 F. The phenol solvent is recovered by separately heating the extract and rafiinate phases to a temperature at which the phenol vaporizes. The hot phenol vapors are taken overhead and are combined, condensed, and returned to a phenol accumulator drum.

A suitable safety valve is installed at a point where the phenol vapors are combined. A safety valve can also be installed contiguous to the phenol treating tower. When the pressure builds up and exceeds the valve setting, the valve will open and discharge the vapors. The vapors are sent to a tank containing relatively cool oil. The vapors enter the tank below the oil level and are condensed on contact with the oil.

Associated with the phenol treating plant are numerous feed and product storage tanks which can be used to coo-l the hot phenol vapors. Also, depending on the ambient temperature, a certain amount of heat may be required to keep such tanks contents pumpalble. To provide storage tank heating requirements, a furnace and a circulating oil system is used. This system provides for indirect heat exchange to heat the stored materials. The circulating oil after heating the storage tank passes into its own storage tank from which it is pumped to the furnace. Thecirculating oil storage tank can also be used for the phenol condensation. After correction of the upset which caused the safety valve to discharge, the oil containing the condensed phenol vapors can be fed to the phenol extract recovery tower to separate the phenol solvent from the oil.

Several advantages are obtained by this invention. If the storage tank of the circulating oil system is used to condense phenol, the cost of the tank is nominal as the tank performs its regular function of storing circulating oil. Condensing the phenol obviates the disposal problem of phenolic waters produced in the conventional phenol blowdown drums using recirculating water. The loss of phenol is entirely eliminated by sending phenol/ oil mixture to the extract recovery section of the phenol plant once the upset condition is corrected. The present invent-ion overcomes the problems of atmospheric pollutron.

The invention is illustrated by the following drawing which is a schematic flow diagram showing the relationship between the phenol tre-ating plant and the circulating oil tank. This invention can be used to prevent escape of any material wihch could escape as hot vapors to the atmosphere upon release of a safety valve. A specific process in which this invention can be used is the treatment of hydrocarbon oils to selectively remove aromatic constituents. Suitable feeds to the phenol treating step may be light lube dist-illat'es, heavy lube distillates, and deasphalted oil stocks. Suitable solvents for extracting other than phenol can be furfural, chlorex, (2,2'-dichloroethyl ether), nitrobenzene, etc. The circulating fluid in the tank heating hot belt system can be any stable hydrocarbon liquid boiling above about 600 F.

The storage tank for the hot oil belt system will pro- Vide for sufiicient circulation of hot fluid to maintain the oils in the other tanks in a pumpable state. For example, oil in the circulating oil storage tank will be at about F. The hot belt furnace will heat this oil to a temperature of about 400 F. which, on heat exchange with the oil in the various feed and product storage tanks, will main tain said tanks at a temperature of about ISO-250 F. The circulating oil will return to the storage tank at a temperature of about 150 F. The pressure in the storage tank for the circulating oil of the hot belt system will be atmospheric; and the tank will be vented to the atmosphere through a suitable stack.

The present invention will be described in conjunction with carrying out phenol treatment of lubricating. oil fractions. In this embodiment, the phenol extraction tower is of conventional design. To protect the treating tower, it is provided with a safety valve set at a predetermined pressure which, in the event of an upset will be opened to release phenol.

The principal units of the phenol treating plant are the treating tower, the extract and recovery towers and furnaces, a phenol condenser, and a phenol solvent accumulator drum.

The principal units of the oil hot belt system are the circulating oil storage tank, a furnace to heat the circulating oil, the various oil storage tanks to which the hot belt system provides heat, means provided by the phenol plant and the hot belt system to convey hot solvent vapors resulting from a safety valve release to the circulating oil storage tank, means for routing the circulating oil from the hot belt system to the phenol extract recovery tower, and means for returning the circulating oil, free of condensed phenol solvent, from the extract recovery tower to the hot belt system.

As previously stated, in the event of a safety blow, the phenol vapor is discharged into the storage tank holding the circulating oil of the tank heating hot belt. Provision is made to insure that there is always a level of oil sufficient to condense the phenol vapor. For example, a tank 25 ft. in diameter with an oil level of about 12 ft. will condense phenol at a rate of about 300,000 lbs/hr. for a short time. Under these conditions, in the event of a phenol safety valve blow, a temperature rise of less than about 50 F. can be expected after five minutes. The phenol vapor escaping on release of the safety valve, in the event of an upset, will be about 450 F. There is enough surface provided in the blowdown tank to condense all the phenol vapor except a small amount which is in equilibrium with the non-condensed stripping gas vapors. Non-condensed gases will comprise primarily the stripping gas and phenol vapor.

Under certain circumstances, the amount of phenol in equilibrium with the noncondensable gases may be large enough so that if the vapors were discharged directly to the atmosphere, they might be in excess of allowable safety limits. Where it is desirable to minimize the ground concentration of phenol vapors to less than 50 parts per million, in a preferred embodiment of the invention, the noncondensed vapors from the blowdown tank will be sent to a small condensable blowdown drum and contacted with water. The water and condensed phenol will be sent to a small tank and later, to recover the phenol, they can be sent to the large oil blowdown tank after it has cooled. As mentioned earlier, the oil-phenol-water mixture in the circulating oil storage tank can be sent to the solvent recovery section of the phenol plant, to recover the phenol.

This invention may be more readily understood by referring to the accompanying drawing, which illustrates a plant suitable for carrying out a preferred embodiment of this invention.

The drawing has been simplified by omitting various pumps, compressors, etc., which do not form part of the present invention.

A feed which is to be phenol extracted will be discussed. The feed enters the bottom of phenol treating tower 1 through line 2 from feed storage tank 3 and is countercurrently contacted with liquid phenol, fed into the top of tower 1 through line 4 from storage drum 14. A rafiinate stream is removed overhead through line 5. The extract phase is removed from the bottom of phenol treating tower 1 through line 6. The extract phase contains most of the phenol solvent and the extracted aromatic hydrocarbons.

The overhead raffinate stream contains a small amount of phenol solvent and most of the nonaromatic constituents of the feed.

The phenol extraction tower 1 is operated at a temperature of about ISO-250 F. and a pressure of about 100- 200 p.s.i.g. The rafiinate stream is fed to rafiinate furnace 7 through line where it is heated to about 685 F., wherein the phenol is partially vaporized. The two-phase liquid-vapor mixture is sent to rafiinate phenol recovery tower 8 through line 9, where the phenol is vaporized and separated from the oil. Phenol solvent recovery in tower 8 is maximized by stripping with hot light stripping gas. The phenol vapor leaves tower 8 via line 10 at a temperature of about 435 F. and a pressure of about 50 p.s.i.g., and is subsequently combined with the phenol from the extract tower 11 in line 12. The phenol in line 12 is condensed by condenser 13 and liquid phenol is introduced into storage drum 14 by line 24.

The raffinate product, virtually free of phenol solvent, is removed from tower 8 by line 15 and can be fed to a storage tank 16.

The extract phase is removed from tower 1 by line 6 and proceeds via valve 17 and line 18 and is heated by extract furnace 19 where the temperature is increased to about 600 F. wherein the phenol is partially vaporized. The extract phase is then fed through line 20 into extract recovery tower 11. The hot phenol vapors are taken overhead through line 21 where they are combined in line 12 with the phenol vapors from raftinate recovery tower 8. Phenol-free hydrocarbons leave tower 11 through line 22.

Extract phase leaves the extract recovery tower and is sent to extract storage tank 41 through lines 22 and 48.

Now, referring to the tank heating system: circulating oil storage tank 26 for the system is about 25 ft. in diameter. A minimum level of about 12 ft. of oil is maintained in the tank. The tank is provided with spider conduits 27 at least 8 ft. below the oil level. Spider 27 is connected through lines 25 and 50 to safety valve 43 and 49 respectively. Circulating oil is fed through line 28 into pump 30 through line 51 to the hot belt system furnace 31. After the oil is heated in the furnace, it is withdrawn through line 32 and fed into lines 33, 34, and 35, into product storage tank 16, extract storage tank 41, and hydrocarbon feed tank 3. In these tanks, the circulating hot oil passes through heating coils to exchange heat with the oils stored therein, maintaining a temperature in the storage tanks of about '-250 F. After heating the storage tanks the circulating oil is returned to tank 26 by lines 36, 37, 38, and 39.

When there is an upset and a release of hot phenol vapors there is a flow of hot phenol vapors at a rate of about 300,000/lbs./hr., for about a 5-minute fiow of hot vapors and about 5 mm. B.t.u.s will be transferred to the oil in tank 26. The increase in oil temperature Will be less than about 50 F. Substantially all of the phenol vapors will be condensed in the oil. The phenol condensed in the circulating oil may be recovered by diverting part of the circulating oil in line 28 throughvalve 40 and line 42 and valve 17 into line 18 and into the phenol extract recovery tower 11. The circulating oil and condensed phenol solvent will first pass through line 18 into extract recovery furnace 19 and through line 20 into phenol extract recovery tower 11. Vaporized phenol will be taken overhead through line 21 and circulating oil withdrawn from the bottom of tower 11 through line'22, be diverted by valve 46 and returned to the circulatin oil system through line 47 and valve 29. a

It is obvious from the above description that this invention provides an efiicient, safe means for condensing hot vapors in the event of safety valve release and in recovering the hot vapors which would otherwise be lost or cause a pollution problem.

The invention is not to be limited to the ples but only by the following claims:

What is claimed is: v

1. An improved process for treating hydrocarbon fractions to selectively solvent extract aromatic constituents from said hydrocarbon fractions wherein the solvent extraction is carried out at elevated temperatures and pressures which comprises that in the event of an upset resulting in the release of hot solvent vapors that the vapors are passed to an oil storage tank containing cool oil and are condensed, said cooling liquid being used as a separate hot circulating fluid for heating a feed hydrocarbon fraction storage tank.

2. The process of claim 1 wherein the solvent vapor condensed in the oil is recovered by running the oil toabove examgether with the condensed solvent through a solvent re covery distillation tower.

3. In the process of solvent treating lubricating oil fractions with a phenol solvent to selectively remove aromatic constituents wherein the lubricating oil fraction is countercurrently contacted with the phenol solvent at an elevated temperature and pressure to produce an extract phase and a rafiinate phase and wherein a hydrocarbon oil is used as a heat exchange medium normally circulating from an oil storage tank as a separate hot belt liquid to supply heat to the liquids in product storage tanks and the oil feed storage tank and said oil storage tank also functioning as a recovery zone for phenol released from the process, the improvement which comprises passing hot phenol vapors released from the process in an upset of conditions into said oil storage tank, condensing the phenol vapors by direct contact with the oil in said oil storage tank, recovering the condensed phenol from said hot belt liquid oil by passing said hot belt liquid oil to said phenol recovery extract tower, separating the phenol from said hot belt liquid oil by distillation and returning the separated hot belt liquid oil to said circulating hot belt liquid oil.

4. In the process of solvent treating lubricating oil fractions with a liquid solvent selected from the group consisting of phenol, furfural, S0 chlorex, nitrobenzene and mixtures thereof, to selectively remove aromatic constituents wherein the lubricating oil fraction is countercurrently contacted with the liquid solvent at an elevated temperature and pressure resulting in an extract phase and raffinate phase, wherein the liquid solvent is recovered as a vapor by separating the extract and raflinate phases and by separately distilling said phases in solvent extract and solvent raffinate recovery towers, and wherein a hydrocarbon oil is used as a cooling liquid only during an upset of conditions in the process, said cooling liquid being stored in a storage tank, the improvement which comprises passing the hot solvent vapors released from a safety valve during an upset in the process into said storage tank and condensing them by direct contact with said cooling liquid therein, recovering the condensed solvent from said cooling liquid by passing said cooling liquid to the solvent recovery extract distillation tower and separating the solvent from said cooling liquid by distillation, returning the separated solvent to the process and returning said cooling liquid substantially free of solvent to said storage tank.

References Cited by the Examiner UNITED STATES PATENTS 1,310,748 7/1919 Fulweiler 20834l 1,680,352 8/1928 Werkenthin 208333 2,299,830 10/1942 Legatski et a1 208-341 2,733,187 l/1956 Basch 208-321 FOREIGN PATENTS 638,201 3/ 1962 Canada.

OTHER REFERENCES Refiner and Natural Gasoline Manufacturer, vol. II, No. 2, pp. 200-201, February 1932.

DELBERT E. GANTZ, Primary Examiner.

ALPHONSO D. SULLIVAN, Examiner.

P. P. GARVIN, H. LEVINE, Assistant Examiners. 

1. AN IMPROVED PROCESS FOR TREATING HYDROCARBON FRACTIONS TO SELECTIVELY SOLVENT EXTRACCT AROMATIC CONSTITUENTS FROM SAID HYDROCARBON FRACTIONS WHEREIN THE SOLVENT EXTRACTION IS CARRIED OUT AT ELEVATED TEMPERATURES AND PRESSURES WHICH COMPRISES THAT IN THE EVENT OF AN UPSET RESULTING IN THE RELEASE OF HOT SOLVENT VAPORS THAT THE VAPORS ARE PASSED TO AN OIL STORAGE TANK CONTAINING COOL OIL AND ARE CONDENSED, SAID COOLING LIQUID BEING USED AS A SEPARATE HOT CIRCULATING FLUID FOR HEATING A FEED HYDROCARBON FRACTION STORAGE TANK. 